Pulse width modulated ("PWM") waveforms are used to turn On and Off the power transistors in a voltage source inverter to control three-phase loads, such as AC motors. The On and Off durations of each transistor correspond to the pulse widths that are applied to the gate or base of the transistor. The On and Off switching of the transistors delivers desired average power and current to the load, e.g., to the AC motor whose current or torque must be controlled. An example of a prior art circuit and method for PWM is shown in U.S. Pat. No. 4,703,245, "Method and Apparatus for Controlling Permanent Magnet Synchronous Motor by Using Pulse Width Modulation," which issued to Sakamoto et al. on Oct. 27, 1987, and was assigned to Fanuc Ltd. This patent relates to a method and apparatus for controlling a permanent magnet synchronous motor, wherein an inverter is controlled in response to PWM signals obtained by comparing a reference carrier wave with signals representing differences between armature winding current command signals of the respective phases and detected armature currents. The arrangement disclosed in this patent is useful to illustrate a basic PWM technique. However, it is an expensive, not integrated solution that uses discrete analog components, various ROM modules and digital to analog converters ("D/As"). It is also aimed at only controlling Permanent Magnet Synchronous motors (or Permanent Magnet Brushless DC motors).
Balanced three phase loads and AC motors typically require ideal sinusoidal current waveforms in each phase. Depending on the PWM generation method that is used, the generated phase current varies from the ideal sinusoidal signal in smoothness, noise and harmonics. Noisy phase currents cause undesired behavior and torque ripples in the operation of the load circuit and motor.
Different modulating schemes, including sinusoidal modulating schemes, may be used to produce PWM waveforms. The common objective of all of these methods is to reduce noise in the load and motor phases, and to increase efficiency of the supply voltage usage in implementing the PWM scheme.
One novel approach that does not use a sinusoidal modulating signal is the space vector PWM method. The space vector PWM method has as its goal the production of an optimal switching pattern of the power transistors such that the phase current noise, i.e., the total harmonic distortion ("THD") is minimized and the output voltage capability of the driver is increased by over 13 percent over sinusoidal modulating schemes.
An example of prior art circuits and methods using the space vector PWM method are shown in U.S. Pat. No. 5,182,701, "Three-Phase PWM Inverter Providing an Improved Output Sinusoidal Waveform," which issued on Jan. 26, 1993, to Mochikawa, et al. and was assigned to Kabushiki Kaisha Toshiba, and in U.S. Pat. No. 5,428,283, "Power Factor Control of Pulse Width Modulated Inverter Supplied Permanent Magnet Motor," which issued on Jun. 27, 1995, to Kalman et al. and was assigned to Alliedsignal Inc.
The '701 patent to Mochikawa et al. relates to an inverter in which six switching elements of an inverter main circuit are controlled to be turned On and Off under a switching pattern in which a voltage space vector composed of adjacent two of six fundamental voltage vectors out of phase from one another by an electrical angle of .pi./3 and a zero vector figures a circular locus with the goal of obtaining a three-phase substantially sinusoidal voltage. Two switching patterns of the same kind are to be formed before and after the formation of the switching pattern corresponding to the zero vector. The switching pattern corresponding to the zero vector is to be held for half of its holding time when the voltage space vector passes an intermediate phase position between the two adjacent fundamental voltage vectors.
The method described in '701 patent focuses on switching losses and minimizing torque ripples. It is an independent, not on-chip integration and CPU software intervention, hardware calculator with large memory. Twelve segments are used in one complete rotation, a 360 electrical degree cycle. The wave form symmetry is not discussed, and the switching pattern will be fixed due to hardware realization. The boundary conditions that may occur during operation have not been addressed clearly. The circuit does not include deadband, it takes 2 input commands, phase command and voltage command related to each other, and provides six output signals.
The '283 patent to Kalman et al. relates to power factor control of a pulse width modulated inverter supplied permanent magnet motor by using Park Vectors (space vectors) for automatically adjusting a pulse width modulating signal for approximately unity power factor, with the goal of avoiding manual adjustment during operation of the motor for power factor changes in accordance with changes in EMF and motor resistance. The '283 patent is directed to power control, however, and is mentioned solely for general background purposes.
Space vector ("SV") methods have been implemented by software using digital processors, microcontrollers, and discrete components. See, for example, "A Novel PWM Scheme of Voltage Source Inverters Based on Space Vector Modulation Strategy," by Stoshi Ogasawara, Hirofuni Akagi, et al., Archiv fur Elektrotechnik 74, pp 33-41 1990; "An Efficient Microprocessor-Based Pulse-Width Modulator Using Space Vector Modulation Strategy, by L. Zhang, C. Wathanasarn and F. Hardan, IEEE, pp. 91-96, 1994; "Variable Structure Approach for Induction Motor Control--Practical Implementation of DSP, by Moon-Ho Kang, Nam-Jeong Kim, et al., IEEE, pp. 50-55 1994; "Sinusoidal PWM Techniques with Additional Zero-Sequence Harmonics, by S. Halasz, G. Csonka and A. A. M. Hassan, IEEE, pp. 85-90 1994; and "Space-Vector PWM Voltage Control with Optimized Switching Strategy, by V. R. Stefanovic, IEEE IAS Annual Meeting, pp. 1025-1033, 1992.
Implementation of the SV PWM method software on a microcontroller or a digital signal processor ("DSP") requires execution of many instructions. The code size and especially the execution time of the software instructions does not satisfy the design constraints of high performance control systems in many applications. In today's control system designs, carrier frequencies of 25 KHz and higher are used for PWM waveform generation. The SV PWM waveform generation methods require the change of switching states of the power transistors up to four times in every PWM period, which would be 40 .mu.sec. for a 25 KHz PWM carrier frequency, for example. Furthermore, the change of switching states are commonly driven by interrupt signals. In microprocessors ("MPs"), microcontrollers ("MCUs") and DSPs, there is a certain delay time for the CPU to recognize the occurrence of an interrupt and to put the software program at the beginning of a proper interrupt service routine. This delay time is frequently called the "Interrupt Latency" of the processor. Handling three to four interrupts, with the attendant delay times, and executing many software codes in every PWM period of 40 .mu.sec. or less makes it a very difficult, and in many applications an impossible task for an MP, an MCU and even a high performance DSP to implement the SV PWM method of waveform generation.
Furthermore, it would be desirable for a Space Vector PWM realization and method to provide software flexibility so that user can optimize the pattern generation according to selected power inverter characteristics, with minimum electrical and acoustic noise that are typically produced in controlling AC motors, and equally distributed power dissipation among the power switches so that switching losses are low, over-stress of transistors are eliminated, and cost per performance is minimized.
Thus, there is a need for an improved SV PWM method to integrate the SV PWM waveform generation on a DSP device that overcomes the difficulties in prior art software implementations of the SV PWM waveform generation methods, and provides reduced system cost, reduced CPU overhead, and at the same time increases system reliability and performance. The present invention provides such a method.